Oxide varistor

ABSTRACT

OXIDE VARISTOR COMPRISING A BASIC COMPOSITION (TOTALING 100 MOL PERCENT) FORMED OF 87 TO 12 MOL PERCENT OF ZNO, 1 TO 30 MOL PERCENT OF SB2O3 AND 12 TO 87 MOL PERCENT OF AT LEAST ONE METAL OXIDE SELECTED FROM THE GROUP CONSISTING OF MGO, COO, NIO, BAO, SRO, CAO, MNO, FEO AND CUO AND AN ADDITIVE CONSISTING OF 0.5 TO 10% BY WEIGHT OF BI2O3 BASED ON SAID BASIC COMPOSITION.

NOBORU ICHINOSE E L 3,836,483

Sept. 17, 1914 0XIDE.-.VARISTOR 4 Sheets-Sheet 2 Filed May 24, 1972 1%! AQ SIO NVOw E c w. m w

(U) BONViSISEIH HONViSISHH Sept. 17, 1974 NOBORU ICHINOSE E AL 3 OXIDE. VARISTOR Filed May 24, 1972 4 Sheets-Sheet 4 FIG. 7

NONLINEAR VOLTAGE COEFFICIENT (0Q) N CONTENT OF BizOs (weight United States Patent M U.S. Cl. 252-519 1 Claim ABSTRACT OF THE DISCLOSURE Oxide varistor comprising a basic composition (totaling 100 mol percent) formed of 87 to 12 mol percent of ZnO, l to 30 mol percent of Sb O and 12 to 87 mol percent of at least one metal oxide selected from the group consisting of MgO, CoO, NiO, BaO, SrO, CaO, MnO, FeO and CuO and an additive consisting of 0.5 to by weight of Blgog based on said basic composition.

This invention relates to a varistor prepared from an oxide semiconductor. Typical known varistors consisting of a semiconductor are SiC varistors. SiC varistors have nonlinear voltage-current characteristics, namely, are sharply reduced in resistance with higher voltage to permit the passage of current therethro'ugh in amounts increased by that extent and have consequently been widely used for absorbing normally high voltage or for stabilization of voltage. In recent years, telecommunication apparatus, for example, has come to be formed of transistors, resulting in the low operating voltage of circuits. Accordingly, there is growing demand for a low voltage (or low resistance) type of varistor.

Generally, the voltage-current characteristics of the varistor may be expressed approximately in the following equation:

1=(V/ C) where:

I=current flowing through the varistor V=voltage across the varistor C-=constant u=nonlinear voltage coeflicient Therefore, the characteristics of the varistor may be indicated by C and a or two other constants which can replace them. Since accurate determination of C presents extreme difiiculties, C is generally substituted by voltage Vc at a .certain current C ma. With the varistor voltage thus designated as Vc, the voltage current characteristics of the varistor may be indicated by Vc and the nonlinear constant oz.

The nonlinearity of the SiC varistor is derived from the voltage sensitivity of the contact resistance of SiC particles. This SiC varistor is generally prepared by mixing SiC powders with porcelain binder material or conductive material like graphite depending on the object intended and sintering the mass at elevated temperatures after it is molded..

As is well known, the varistor is desired to have as large a nonlinear voltage coefiicient a as possible. The aforesaid SiC varistor has a relatively large value (about 3 to 7) of 0c and is stabilized in other electrical properties, so that it may be deemed as adapted for practical application. Nevertheless, the SiC varistor has the drawback that it presents ditficulties in being developed into a low voltage type. To obtain a low voltage SiC varistor, there have been made attempts to form the varistor into a disc shape in order to reduce its resistance or incorporate conductive material like graphite in order to decrease 3,836,483 Patented Sept. 17, 1974 its specific resistivity. In the former attempt, the thinning of the SiC varistor poses problems with its mechanical strength. And in the latter attempt, incorporation of graphite which essentially lacks nonlinearity in connection with resistance will eventually reduce the nonlinear voltage coeflicient of a resultant varistor, thus naturally imposing limits on the formation of a low resistance varistor. The SiC varistor presents difliculties in having its voltage Vc reduced to, for example, less than 10 volts and consequently is unsuitable for low voltage application.

Due to the development of transistor circuits, however, recent demand for varistors tends to increase with respect to a type having a voltage of less than 10 volts. Further, attempts are being made to render an apparatus using a varistor more compact and efficient and in consequence the varistor is desired to display high performance by a simple circuit arrangement. This holds true not only with low voltage application but also with the voltage level to which the SiC varistor has heretofore been applied.

To date, there has been developed a Zn() oxide varistor whose preparation is characterized by adding 0.1 to 10 atomic percent of MgO to ZnO, sintering the mass in the air at temperatures of 900 to 1500 C. and using glass of lead borosilicate as an electrode. However, this type of varistor indicates a nonlinear voltage coeificient u of 6 at most and is not deemed fully available for practical application. In addition, the US. Pat. 3,598,763 discloses a manganese-modified zinc oxide varistor, which is neither considered to have a fully large nonlinear voltage coefficient.

It is accordingly the object of this invention to provide in view of the aforementioned circumstances a high performance varistor which permits easy voltage control and has a sufliciently large nonlinear voltage coefiicient oz for use with high voltage circuits of more than 300 volts included in such apparatuses as colour television receiving sets and electronic ranges.

The varistor of this invention comprises a basic composition (totaling mol percent) formed of M01 percent ZnO 87 to 12 MeO 12 to 87 SbgO 1 to 30 where:

MeO=one selected from the group consisting of BaO SrO, CaO, MgO, CoO, NiO, MnO, FeO and CuO and a minor component formed of 0.5 to 10% by weight of Bi O based on said basic composition.

This invention can be more fully understood from the following detailed description when taken in connection with reference to the accompanying drawings, in which:

FIGS. 1 to 6 indicate the voltage-current characteristics of the basic composition of an oxide varistor according to this invention: FIGS. 1 to 3 are curve diagrams showing the relationship of the content of Sb O defined with respect to the prescribed proportions of ZnO and MeO and the resistance of the varistor; and FIGS. 4 to 6 are curve diagrams showing the relationship of the mol ratio of ZnO to MeO (with the proportion of Sb O fixed) and the resistance of the varistor;

FIG. 7 is a curve diagram indicating the relationship of the proportion of Bi O and the nonlinear voltage coefiicient of the oxide varistor of this invention by way of illustrating its properties; and

FIG. 8(a) is a sectional view schematically showing the arrangement of sintered crystals of the oxide varistor of this invention;

FIG. 8(b) is a sectional view schematically showing the arrangement of the sintered SiC crystals of the prior art SiC varistor.

The above-mentioned oxide varistor of this invention may by prepared, for example, in the following manner. Raw oxides accurately weighed out to form prescribed proportions are mixed in a ball mill, presintered at relatively low temperatures as 600 to 850C. and later pulverized, for example, in a ball mill into extremely fine powders. It will be apparent that the raw materials used may consist of other metal compounds convertible to oxides with heat, for example, hydroxides, carbonates and oxalates of metals. The powders obtained are mixed with a binder, for example, polyvinyl alcohol. The mass is molded at a pressure of 100 to 2000 kg./cm. into a disc about 8 mm. in diameter and about 1 mm. thick, followed by sintering at temperatures of 1000 to 1400 C. in an electric furnace. Said sintering may generally be carried out in the air and a maximum sintering temperature generally has only to be maintained for 1 to hours.

There will now be given the reason why the proportions of the basic constituents of an oxide varistor according to this invention have been limited to the aforementioned ranges. The contents of said basic constituents, namely, ZnO, MeO and Sb O have been found to have the undermentioned relationship with the resistance of said varistor. FIGS. 1 to 3 present variations in the resistance of a varistor prepared with the mol ratio of ZnO to MeO fixed at 2.0 and the proportion of Sb O varied. Referring to FIG. 1, the curve (a) denotes the case Where Me represents Ba, the curve (b) the case where Me represents Sr and the curve (c) the case where Me represents Ca. Referring to FIG. 2, the curve (d) shows the case of Me=Mg, the curve (e) the case of Me=Co and the curve (1) the case of Me=Ni. Referring to FIG. 3, the curve (g) indicates the case of Me=Mn, the curve (h) the case of Me=Fe, and the curve (j) the case Me=Cu. As apparent from these figures, where the proportion of Sb O rises above 1 mol percent, the resulting varistor is reduced in resistance and adapted for practical application. In contrast, where the proportion of Sb O exceeds 30 mol percent, the resulting varistor will have an unduly large resistance or lose readiness for sintering, though it may not raise any problem with resistance, thus eventually failing to serve practical application.

Further, determination was made of the resistance of a varistor prepared with the mol ratio of ZnO to MeO varied and the proportion of Sb O fixed to mol percent, the results being presented in FIGS. 4 to 6. Referring to FIG. 4, the curve (a) denotes the case of Me=Ba, the curve (12) the case of Me=Sr and the curve (0) the case of Me=Ca. In FIG. 5, the curve (d) indicates the case of Me=Mg, the curve (e) the case of Me=Co and the curve (1) the case of Me=Ni. Referring to FIG. 6, the curve (g) shows the case of Me=Mn, the curve (h) the case of Me=Fe, and the curve (j) the case of Me=Cu. It is seen from these figures that in the case where the proportion of ZnO included in the ZnO-MeO-Sb O system falls outside of the range of 87 to 12 mol percent or in the case where the proportion of MeO included in said system departs from the range of 12 to 87 mol percent, then such system will become unsuitable as the basic composition of a varistor according to this invention due to the occurrence of high resistance.

There will now be described the reason why the proportion of the additive of Bi O is limited to 0.5 to 10% by weight based on the basic composition formed of a system. Determination was made of the nonlinear voltage coeificient a of a varistor prepared by adding varying amounts of Bi O to a basic composition formed of, for example, 60 mol percent ZnO, 27 mol percent of MgO and 13 mol percent of Sb O Then the nonlinear coeflicient presented such variation as illustrated in FIG. 7. This figure shows that addition of Bigoa in amounts falling outside of the aforesaid range failed to provide a large (a 7) nonlinear voltage coefficient. Where MgO was replaced by other MeO, there was observed the same tendency as in FIG. 7.

The voltage-current characteristics of the oxide varistor according to this invention did not vary in any form of its composition, provided the constituents were incorporated in the prescribed proportions, or even when the electrode was formed of silver or In-Ga alloy.

Though not clearly defined, the reason why the oxide varistor according to this invention displays good voltagecurrent characteristics is supposed to originate with the following facts. This varistor has such a structure as schematically illustrated in FIG. 8(a). Like the SiC varistor whose structure is schematically shown in FIG. 8(b), the varistor of this invention supposedly derives its nonlinear characteristics from the particular phases of the boundaries between the sintered fine crystals of raw materials used and is constituted by innumerable agglomerations of said boundary phases. Referring to FIGS. 8(a) and 8(b), numerals (3) and (3') respectively represent paired electrodes, (1') SiC particles and (2) a binding agent. However, the varistor of this invention is widely different from the conventional SiC varistor in that the nonlinearty characteristics of the former varistor originate in the boundary zones between individual grains of the sintered materials, that is, in the contacting zones of the grains, in contrast to SiC varistors whose characteristics originate in contact resistance. Said difference may be deemed to have a prominently favorable effect on the voltage-current characteristics of the varistor of this invention.

The SiC varistor indeed resembles the present varistor in that the voltage of the SiC varistor can be limited within a considerably broad range, namely, its voltage can be adjusted to any desired level by controlling a number of serially arranged nonlinearity boundaries or the width thereof. But the SiC varistor is distinctly different from the varistor of this invention whose voltagecurrent characteristics and the size of the crystal particle can be relatively freely varied. With the SiC varistor, the.

size of its crystals is primarily determined by the SiC particles constituting the main raw material which do not widely vary even by the sintering process. With the varistor of this invention, however, the powders of start ing raw materials have a particle size ranging preferably approximately between 0.1 and 1 micron. Moreover, said particle size can be increased by sintering to several or scores of microns. The present varistor has the further advantage that not only the particle size but also the specific resistivity of the fine grains of the raw material can be controlled by varying the composition, the kind of additives or the sintering conditions, thereby rendering the varistor more adapted for practical application. In contrast, the SiC varistor does not display much desired nonlinearity characteristics, which is supposed to originate from the fact that SiC itself does not have an appreciably low specific resistivity and said resistivity can not be easily controlled.

As mentioned above, the fine grains of the varistor of diode, though the former is diiferentfrom the latter in that it indicates nonpolar and symmetrical voltagecurrent characteristics.

This invention will be more fully understood by reference to the examples which follow:

9 varistor voltage is less than 0.005%, which is much smaller than those known of a Zener diode i.e., about 0.1%, or of SiC varistor i.e., 0.1 to 0.2%. It will be also found that its surge current is very large. It is more than 100 times as high as that for a Zener diode.

TABLE 2 Var'istor voltage variation with temperature (percent/ C.)

Surge current (AJem?) Example was What we claim is:

1. An oxide varistor having a nonlinear voltage co efficient greater than 7 comprising a basic composition (totaling 100 mol percent) consisting of 87 to 12 mol percent of ZnO, 1 to mol percent of Sb O and 12 to 87 mol percent of at least one metal oxide selected from the group consisting of MgO, CoO, NiO, BaO, SrO, CaO, MnO, FeO and C110, and an additive consisting of 0,5 to 10 percent by weight of Bi O based on said basic composition.

References Cited UNITED STATES PATENTS JOHN D. WELSH, Primary Examiner US. Cl. X.R. 338-21; 252521 

